Recording performance for magnetic disks are determined by three basic characteristics--narrow PW50, high overwrite, and low noise. PW50 is the pulse width of the bits expressed in either time or distance, defined as the width of the pulse at half-maximum. Having a narrower (and more well-defined) pulse allows for higher recording density. A wide PW50 means that the bits are crowded together, causing them to interfere with each other. This interference is termed inter-symbol interference. Excessive inter-symbol interference limits the packing density of bits in a given area.
Conventionally, there are number of media factors which affect PW50. In order to achieve narrow PW50, the coercivity ("Hc") of the media must be high. However, if Hc is too high, the head field will have a difficult time saturating the media, resulting in poor overwrite. Overwrite ("OW") is a measure of the ability of the media to accommodate overwriting existing data. That is, OW is a measure of what remains of a first signal after a second signal (for example of a different frequency) has been written over it on the media. OW is poor when a significant amount of the first signal remains. OW is generally affected by Hc, thickness, and the hysteresis loop squareness of the film.
PW50 may be reduced by using a thinner magnetic film. Another means of reducing PW50 is to increase hysteresis loop squareness, and narrow the switching field distribution, as described by William and Comstock in "An Analytical Model of the Write Process in Digital Magnetic Recording," A.I.P. Conf. Proc. Mag. Materials, 5, p. 738 (1971). Hysteresis loop squareness ("S") has several components, including coercivity squareness ("S*") and remnant coercivity squareness ("S*rem").
Noise performance of a magnetic film can be defined in terms of read jitter and write jitter. In peak-detection type recording channels, Noise, together with inter-symbol interference, contributes to the uncertainty in the location of the individual bits, which cause the data to be read with some displacement in timing from that which is expected. This displacement is referred to as bit shift. The bit shift needs to be reduced to a minimum for a given timing window of the bit in order to assure accuracy in reading the bit.
Read jitter is primarily determined by the amount of signal available from the bit, and the electronic noise in the channel. A thicker magnetic film will typically provide reduced read jitter. Unlike read jitter, write jitter is determined by the intrinsic noise of the film. Intrinsic media noise has been theoretically modeled by Zhu et al. in "Micromagnetic Studies of Thin Metallic Films", J. Appl. Phys., vol. 63, no. 8, p. 3248 (1988), which is incorporated by reference herein. Chen et al. describe the source of intrinsic media noise in "Physical Origin of Limits in the Performance of Thin-Film Longitudinal Recording Media," IEEE Trans. Mag., vol. 24, no. 6, p. 2700 (1988), which is also incorporated by reference herein. The primary source of intrinsic noise in thin film media is from the interparticle exchange interaction. In general, the higher the exchange interaction, the greater the noise.
The noise from interparticle exchange interaction can be reduced by isolating the individual particles. This may be accomplished by spacing the grains apart from one another, or by interposing a non-magnetic material or insulator at the grain boundaries as described by Chen et al. in the aforementioned "Physical Origin of Limits in the Performance of Thin-Film Longitudinal Recording Media." The amount of separation needs to be only a few angstroms. There is another interparticle interaction, called magnetostatic interaction, which acts over a much greater distance between particles as compared to the exchange interaction. Reducing the magnetostatic interaction does reduce intrinsic media noise slightly. However, the effects of magnetostatic interaction actually improve hysteresis loop squareness and narrow the switching field distribution, and hence improve PW50 and OW Therefore, magnetostatic interaction is generally tolerated.
In order to obtain the best performance from the magnetic media, each of the above criteria--PW50, noise and OW--must be optimized. This is a formidable task, as each of these performance criteria are inter-related. For example, obtaining a narrower PW50 by increasing the Hc will adversely affect OW, since increasing Hc degrades OW. A thinner media having a lower remnant magnetization-thickness product ("MrT") yields a narrower PW50, however the read jitter increases because the media signal is reduced. Increasing squareness of the hysteresis loop contributes to narrower PW50, but generally increasing squareness increases noise. Thus, the amount that PW50 may be narrowed is limited by the increase in noise. Providing a mechanism for separating or isolating the grains to break the exchange coupling can effectively reduce the intrinsic media noise. Noise is improved by eliminating the interparticle exchange interaction. A slight further reduction of noise is possible by reducing magnetostatic interaction, but this reduces the hysteresis loop squareness and increases the switching field distribution, which degrades PW50 and OW.
In order to obtain the optimum media performance, the MrT must be reduced for better OW and PW50, but still retain sufficient signal to maintain acceptable read jitter. This is principally accomplished by reducing the film thickness (thereby reducing the space loss between the recording head pole tip and the media), and using an alloy having a higher saturation magnetization ("Ms").
Therefore, an optimal thin film magnetic recording media for high density recording applications, i.e., that can support high bit densities, requires low noise without sacrificing the switching field distribution, S*, and S*rem. Recording density can then be increased since bit jitter is reduced. In order to achieve the best compromise in performance, the individual grains of the magnetic film must be isolated to eliminate the exchange interaction, and grains must be uniform and have a tight distribution of sizes to minimize intrinsic media noise while maintaining high hysteresis squareness.
One type of magnetic media which has allowed optimizing certain of the above performance criteria is based on alloys of cobalt (Co) and platinum (Pt). CoPt is typically alloyed with nickel (Ni), chromium (Cr), etc. Attributes of CoPt alloys have been described by Murdock et al. in "Roadmap to 10 Gb/in.sup.2 Media: Challenges", IEEE Trans. Mag., 1992, page 3078, by Opfer et al. in "Thin Film Memory Disk Development", Hewlett-Packard Journal (Nov. 1985), and by Aboaf et al., in "Magnetic Properties and Structure of Co--Pt Thin Film", IEEE Trans. Mag., page 1514 (1983), each incorporated herein by reference. Increasing storage capacity demands and performance requirements have motivated a search for ways to improve Co--Pt based alloys.
As stated above, a high Hc film will produce a narrow PW50, thus permitting an increase in storage density. One method of increasing Hc involves increasing the atomic percent ("at. %") of platinum in the film. This approach is described in Opfer et al., "Thin-Film Memory Disk Development" (referred to above). However, it is known that as the platinum content increases, SNR decreases due to a reduction in signal amplitude without a commensurate decrease in media noise.
In order to decrease the media noise, it is also known to introduce oxygen into the magnetic film in a concentration of 5 to 30 at. %, as taught by Howard et al. in U.S. Pat. No. 5,066,552, incorporated by reference herein. However, as pointed out by Howard et al. in said patent, introducing oxygen decreases both Hc and S*.
Another approach to increasing Hc, as discussed by Maeda in "Effects of Nitrogen on the High Coercivity and Microstructures of Co--Ni Alloy Films," Journal of App. Phys., vol. 53, no. 10, pp. 6941-6945 (October 1982), involves depositing a thin film magnetic media by sputtering in an ambient of argon and nitrogen. An increase in the nitrogen gas concentration in the chamber (e.g., about 24% by volume) is shown in this reference to be accompanied by an increase in Hc. However, magnetic films produced by this method exhibit a decrease in Ms. Also, the film produced by this method is not ferromagnetic as deposited. This method requires the additional step of annealing the deposited film at a relatively high temperature to diffuse large amounts of the nitrogen out of the cobalt film, thus rendering the film ferromagnetic.
Current and future demands of high-density magnetic media are foreclosing the opportunity for a trade off between Hc, Ms, SNR, etc. Therefore, there is at present a need in the art for a method of increasing the coercivity of a thin film magnetic alloy while yielding a high degree of squareness, high SNR, high overwrite, and low PW50.